This invention relates to far infrared (FAR) lasers which are optically pumped by CO.sub.2 lasers. Such systems are described in the articles: "c w Submillimeter Laser Action in Optically Pumped Methyl-Fluoride, Methyl Alcohol, and Vinyl Chloride gases" by Chang et al, pp 249-251, in Applied Physics Letters, Volume 17 No 6, Sept. 15, 1970; "Millimeter and Submillimeter Wave Laser Action in Symmetric Top Molecules Optically Pumped via Parallel Absorption Bands." by Chang et al, pp 103-105, in Applied Physics Letter, Volume 19, No. 4, Aug. 15, 1971; "Submillimeter Lasers Optically Pumped Off Resonance" by Fetterman et al, pp 156-159, in Optics Communications, Volume 6, No. 2, Oct. 1972; "New Submillimeter Laser Lines in Optically Pumped Gas Molecules", by Wagner et al, pp 46-47, Volume 8, No. 1, May 1973; "Waveguide Laser for the Far Infrared (FIR) Pumped by a CO.sub.2 Laser", by Hodges et al, pp 252-253, in Applied Physics Letters, Volume 23, No 5, Sept. 1, 1973 and "Low-Threshold, C W Submillimeter and millimeter Wave Laser Action in CO.sub.2 Laser Pumped C.sub.2 H.sub.4 F.sub.2, C.sub.2 H.sub.2 F.sub.2, and CH.sub.3 OH" by Hodges et al, pp 1159-1160, in Journal of Quantum Electronics Volume 9, No 12, Dec. 1973.
In the normal state of the art optically pumped far infrared lasers, the CO.sub.2 laser is separate from the far infrared cavity, and the CO.sub.2 power is focused into the far infrared cavity to excite the molecules by optical energy transfer. The CO.sub.2 power is coupled into the far infrared cavity by means of a hole, in the mirror at one end. A hole in the mirror at the other end is used to extract the far infrared power from the cavity. In such systems, only the CO.sub.2 laser energy that enters the far infrared cavity is available for exciting the far infrared molecules.